1. Field of the Invention
The present invention relates generally to a handoff method in a mobile communication system supporting an ALL-IP network, and in particular, to a handoff method for removing an overhead of an ALL-IP-based voice transmission frame.
2. Description of the Related Art
Recently, in order to solve an inefficiency problem of radio resources during circuit switching in an IMT-2000 (International Mobile Telecommunication-2000) network supporting both a packet network and a circuit network, many attempts have been made to integrate next generation mobile communication networks into a packet-based IP (Internet Protocol) network, called an ALL-IP network, by major next generation mobile communication standardization organizations including 3GPP (3rd Generation Partnership Project) and 3GPP2 (3rd Generation Partnership Project 2). The ALL-IP network has an open-type data network structure, so it can effectively support future IP-based services and enable integrated, low-cost maintenance using IP. However, the ALL-IP network wastes bandwidths of a radio link due to an excessive traffic overhead occurring in the radio link. Specifically, in a voice service, the most fundamental service of the mobile communication system, an excessive overhead is generated in order to transmit a small frame.
FIG. 1 illustrates a structure of an ALL-IP network based on a third generation mobile communication system. Referring to FIG. 1, an ALL-IP network includes a mobile station (MS) 100, base transceiver systems (BTSs) 110-1 and 110-2, a radio network controller (RNC, also known as BSC) 120, a core network (CN) 130, and a public network (including PSTN (Public Switched Telephone Network) and Internet) 140. The BTSs 110-1 and 110-2, and the RNC 120 support a radio access service. The CN 130 connects the RNC 120 to the public network 140.
In the ALL-IP network, a 40-bit overhead including an RTP (Real Time Protocol) header, a UDP (User Datagram Protocol) header and an IP header is generated on a radio link between the MS 100 and the BTS 110. The 40-bit overhead represents an overhead excluding a channel overhead in a physical layer. Meanwhile, in order to perform routing and other various functions in the ALL-IP network, a voice frame, while it passes through an RTP stack and a UDP stack, adds an RTP header and a UDP header thereto. Further, an additional overhead may be optionally added to the voice frame. As a result, an actual size of the voice frame on a radio link becomes less than 50% of the whole traffic, causing a waste of bandwidths of the radio link. Such a phenomenon becomes more severe when a soft handoff in which the MS 100 occupies two or more radio channels occurs between the BTSs.
FIG. 2 illustrates a soft handoff procedure in which an MS occupies 2 channels. Referring to FIG. 2, a voice frame transmitted through downlink (or forward) and uplink (or reverse) radio channels between the MS 100 and the two BTSs 110-1 and 110-2 also includes a 40-bit overhead for voice channel transmission.